Overall, electromechanical scooter starter solenoids are cheap enough — but the downside is that they’re not very reliable. The contact resistance increases over time, the coil can be open-circuited due to the vibration, and sometimes the power contacts weld up. One solution is to replace them with a solid state relay. In DC mode, we’ll need to use a MOSFET transistor.
As is often the case in automotive systems, the supply negative is connected to the chassis ground, which means we’ll need to use a P-channel MOSFET. The current to be switched is relatively high, between 55 and 100 A (depending on engine capacity and compression), so we need a transistor with a very low RDS(on) capable of carrying a large IDS. Since the starter is a DC motor with brushes, it generates considerable voltage spikes that are quite destructive for the driving device, hence the need to protect everything very well.
A look at the wiring diagrams for various scooters reveals that the safety switch on the brake (which has to be applied first) supplies +12 V, but the starter button (to be operated next) connects to ground. One simple solution is to use an optoisolator. While we’re on the subject, let’s just note that this technique means this circuit can be used for many other applications too. And finally, the circuit must be ‘Plug-nPlay’, i.e. usable with the original connector, thereby limiting the circuit dimensions to 50 × 50 mm. Building a PCB capable of handling a current of 70 A needs a few calculations. The resistance RT of a copper track with thickness E of 35 µm (0.035 mm) with length L and width W is calculated from
RT = 1.7 × 10-5 × L / (E × W) [Ω]
where E, L, and W are in mm, and T = 25 °C). The component positions mean our tracks can be 15.25 × 44 mm, thus each track represents 1.4 mΩ, or 0.7 mΩ if we use a double-sided board. At 75 A, the total voltage drop will be around 100 mV and the power dissipated 7.5 watts. The SUP75P03-07-E3 MOSFET from Vishay Siliconix (Farnell part no. 1794812) offers an RDS(on) of 7 mΩ at 75 A, i.e. 3.5 mΩ if we put two in parallel. In this case, the voltage drop is 0.263 V and the power dissipated in each transistor is around 10 watts. The end result is that we get an overall voltage drop of around 360 mV and a total dissipation of around 27.5 watts.
Let’s take a look now at the circuit diagram. On the left, everything within the dashed rectangle corresponds to the original wiring of the majority of Chinese scooters. R1 sets the current in the 4N28 optoisolator LED to around 25 mA and R2 biases the base of the phototransistor. The phototransistor collector is connected directly to the gates of the two MOSFETs T1 wired in parallel. At rest, the MOSFETs are held ff by R3, but start to conduct when both contacts S1 and S2 are made, thanks to D3 and the low impedance of the starter motor. Once the starter turns, the charge on C2 ensures that the circuit will continue to function.
Components C1, D1, C2, D2, and D3 protect the circuit against the interference produced by a load that is anything but purely resistive. Tests and measurements have been carried out on a scooter using a GY6 engine type CJ12M. The average consumption was 53 A: 49 A at bottom dead center (minimum compression) as against 57 A at top dead center (maximum compression). The voltage drop measured at the circuit terminals was strictly identical to the theoretical value. After three hours’ testing, at a rate of one start every five minutes, no heating was detected.